It is known to check the operability of a tank-venting system by checking, when operating an internal combustion engine with a lambda controller at a low load, whether a lean correction by the lambda controller is necessary on the basis of the gas supplied to the engine from the tank-venting system. This method is insofar unreliable in that it is quite possible that the fuel in the tank will hardly vaporize, this having the consequence that, during regeneration of the adsorption filter of the system, the air drawn by suction through this filter will not be charged with fuel. For this reason, the lambda controller will then have to perform a correction in the rich direction in order to add fuel to the air supplied from the tank-venting system.
The California Environmental Authority (CARB) therefore proposed in 1989, in the context of a set of regulations, to determine, with the aid of a filling level sensor and a temperature sensor in the tank, whether the motor vehicle to which the system is fitted has been refuelled and whether the fuel introduced into the tank is sufficiently warm to allow the assumption that vaporizing of the fuel is occurring. If the signals from these measuring sensors in the tank indicate that an inflow of fuel vapor into the tank-venting system is to be expected but nevertheless no lean correction is detected by the lambda controller, this is to be taken as an indication of non-operability of the tank. This process has the disadvantage that, when the chosen temperature above which vaporizing of the fuel is assumed is relatively high, no check of the operability can be carried out for relatively long periods of time in cold weather, when the fuel is cold. If the mentioned temperature threshold is set lower, there is the risk that an incorrect statement regarding the operability of the system will be made, namely, if no fuel vapor escapes because essentially fully vaporized fuel has been introduced into the tank. Due to the measurement data from the sensors in the tank, vaporization of the fuel is then erroneously assumed whereas in fact it does not occur, and it is accordingly impossible for a lean correction to be performed by the lambda controller.
U.S. patent application Ser. No. 768,973, filed Oct. 8, 1991, describes tank-venting system which has a differential pressure sensor on the tank and an adsorption filter with a ventilation line which can be blocked. In order to check the operability of the tank, this ventilation line is blocked and a check is then carried out with the tank-venting valve open to determine whether a vacuum builds up in the tank relative to the ambient pressure. If this is the case, the system is deemed to be operational. In one embodiment of the method, provision is made to test for a lean correction by the lambda controller when the tank-venting valve is opened if an overpressure in the tank has been detected beforehand, that is, while the tank-venting valve is still closed.
Another earlier application (DE-A-41 09 401) describes a method for checking the operability of a tank-venting system and, in accordance with this method, while the tank-venting valve is open, a check is simultaneously made to determine whether a lean correction is performed by the lambda controller and whether a tank differential pressure greater than a predetermined threshold is detected. If at least one of the two conditions is fulfilled, the system is deemed to be operative.
Despite these known and proposed tank-venting systems and methods and arrangements for checking the operability of the systems, there continues to be the desire to provide improved methods and arrangements of this kind.